1. Field of the Invention
The present invention relates generally to the field of vertical cavity surface emitting lasers. More specifically, it relates to current confinement structures used in vertical cavity surface emitting lasers.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. While there are many variations of VCSELs, one common characteristic is that they emit light perpendicular to a wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific characteristics.
VCSELs include semiconductor active regions, which can be fabricated from a number of material systems, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure, and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD) or by molecular beam epitaxy (MBE).
FIG. 1 illustrates a typical VCSEL 10. As shown, an n-doped gallium arsenide (GaAS) substrate 12 has an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the GaAS substrate 12, and an n-type graded-index lower spacer 18 is disposed over the lower mirror stack 16. An active region 20, usually having a number of quantum wells, is formed over the lower spacer 18. A p-type graded-index top spacer 22 is disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-type conduction layer 9, a p-type cap layer 8, and a p-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonant at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40 that provides current confinement. The insulating region 40 is usually formed either by implanting protons into the top mirror stack 24 or by forming an oxide layer. The insulating region 40 defines a conductive annular central opening 42 that forms an electrically conductive path though the insulating region 40.
In operation, an external bias causes an electrical current 21 to flow from the p-type electrical contact 26 toward the n-type electrical contact 14. The insulating region 40 and the conductive central opening 42 confine the current 21 such that the current flows through the conductive central opening 42 and into the active region 20. Some of the electrons in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the p-type conduction layer 9, through the p-type cap layer 8, through an aperture 30 in the p-type electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a typical VCSEL, and that numerous variations are possible. For example, the dopings can be changed (say, by providing a p-type substrate 12), different material systems can be used, operational details can be tuned for maximum performance, and additional structures, such as tunnel junctions, can be added.
While generally successful, VCSELs have problems. For example, in some applications it is important for a VCSEL to emit light in the fundamental mode. That is, light with a single unimodal spatial mode (from only one area of the VCSEL) and with a single spectral content. However, prior art insulating regions 40 are less than optimal in producing single mode light. To understand why this is so, the insulation region 40 needs to be understood in more detail.
As noted, the insulating region 40 and the central opening 42 act as a current guide into the active region. Also as noted, the insulating region is usually produced either by implanting protons or by forming an oxide layer. Proton implantation is described by Y. H. Lee et al., Electr. Lett., Vol. 26, No. 11, pp. 710-711 (1990) and by T. E. Sale, Vertical Cavity Surface Emitting Lasers, Research Press Ltd., pp. 117-127 (1995), both of which are incorporated by reference. Oxide layers are taught by D. L. Huffaker et al., Appl Phys. Lett., Vol. 65, No. 1, pp. 97-99 (1994) and by K. D. Choquette et al., Electr. Lett., Vol. 30, No. 24, pp. 2043-2044 (1994), both of which are incorporated by reference.
Ion-implanted VCSELs are typically fabricated using a single energy proton implant in the shape of an annular ring to define a current aperture (multiple implant energies are used to electrically isolate the entire VCSEL). That proton implantation creates structural defects that produce a relatively high resistance annular structure having a conductive core. While the high resistance annular structure effectively steers current through its conductive core and into the active region, ion implantation does not produce significant optical guiding. The result is that ion implantation is effective at steering current into the active region, but ineffective at limiting the optical modes of the laser. Thus, prior art ion implanted VCSELS tended to operate with multiple lasing modes.
In contrast, VCSELs that use oxide current confinement regions benefit from the oxide layer's optical index of refraction, which is about half that before oxidation. This forms an optical guide that tends to provide transverse mode optical confinement. However, because of the distributed nature of the series resistance, oxide VCSELs have the highest P-N junction current density and the highest optical gain at the edge of the cavity. This current distribution encourages the formation of higher order spatial modes, particularly at large bias currents. While the transverse mode optical confinement suppresses undesirable higher order optical modes, in the prior art to obtain single fundamental mode operation an oxide optical current confinement region had to have such a small aperture that light from the VCSEL was severally reduced.
Oxide VCSELs (those that use oxide current confinement) typically include an AlGaAs layer with a high aluminum content (97-98%). Such a high aluminum content structure tends to oxidize much more rapidly than the material layers used to form the rest of the P-type DBR mirror (which is typically 85% Al and 15% Ga). To fabricate the oxide current confinement, reactive ion etching is used to form trenches to the edge of a high Al content layer. Oxidation then forms about a 10 micron deep oxide structure in the high Al content layer, while forming less then a 1 micron deep oxide structure in the adjacent layers. The high Al content layer oxidizes with a complex aluminum oxide that is not only an electrical insulator, but also which occupies about the same space as the layer before oxidation.
Because oxide VCSELs and ion-implanted VCSELs have different characteristics, VCSEL designers have had to chose from among competing features, high output power with higher order spectral modes (oxide VCSELs), or lower output power but with fewer spatial modes (ion implanted VCSEL). Therefore, a new technique of forming VCSELs with the benefits of both ion implanted VCSELs and oxide VCSELs would be beneficial.